1. Field of the Invention
The present invention relates to a thermal head for us with a thermal printer, and more particularly, to a thermal head having an improved printing quality and printing lifetime, and a manufacturing method thereof.
2. Description of the Prior Art
In a conventional thermal head, in general, a glaze heat insulation layer is formed on an upper surface of an alumina substrate, and a plurality of heating resistors are arranged in series on an upper surface of the glaze heat insulation layer. Heating portions formed on respective heating resistors are selectively made to generate heat so as to perform a thermal transfer of ink on a thermal transfer ribbon to plain paper, thus enabling the printing of given letters or given images to the plain paper or to directly perform the printing to thermo-sensitive paper.
To explain such a conventional thermal head in conjunction with FIG. 3, a glaze layer 2 having a bulging portion 2a is formed at a position close to an end of a heat radiation substrate 1 made of aluminum or the like.
To an upper surface of the glaze layer 2, a film made of Ta-SiO2 or the like, is laminated by sputtering or the like and the film made of Ta-SiO2 or the like is subjected to patterning by photolithography to form a heating resistor 3.
An electrode 4 for supplying electric energy to the heating resistor 3 which has a thickness of approximately 2 xcexcm is laminated to an upper surface of the heating resistor 3 by sputtering any one of aluminum, copper, gold and the like or by other techniques. Then, the electrode 4 is subjected to patterning to form a common electrode 4a and an individual electrode 4b by means of a photolithography technique.
Then, at a portion on the heating resistor 3 which is sandwiched by respective ends of the common electrode 4a and the individual electrode 4b, a heating portion 3a is formed at a given interval.
Further, on respective upper surfaces of the common electrode 4a, the individual electrode 4b and the heating resistor 3, a protective layer 5 made of hard ceramic is formed so as to prevent oxidization or wear of the heating resistor 3 or respective electrodes 4a, 4b thus enhancing the durability or the lifetime of the thermal head at the time of printing.
Then, by selectively supplying electric power to the electrode 4 in response to printing information, the heating portion 3a selectively generates heat such that the thermo-sensitive paper is colored or ink of an ink ribbon is transferred to plain paper or the like so as to print-given letters or images.
However, the respective electrodes 4a, 4b of the abovementioned conventional thermal head are formed such that the bodies 4a, 4b have a large film thickness of approximately 2 xcexcm so as to reduce the conductive resistance which is generated at the time of feeding power whereby the lowering of the printing quality and the printing thermal efficiency can be obviated.
Accordingly, the respective electrodes 4a, 4b and the heating portion 3a are formed in a stepped shape such that steps 5a are formed on the protective layer 5 on the respective electrodes 4a, 4b and the heating portion 3a. Here, dregs and fine dusts which are generated at the time of printing are gathered at the steps 5a so that there arises a problem that the printing quality and the thermal efficiency are lowered.
Further, the respective electrodes 4a, 4b, in general, are often made of a soft material such as aluminum which is inexpensive, exhibits excellent workability and favorable conductivity. However, a contact pressure force which brings the heating portion 3a of the thermal head into contact with a platen (not shown in the drawing) is repeatedly applied to the heating portion 3a at the time of printing. Accordingly, with respect to the electrode 4 which is formed of the soft material such as aluminum, ends of the electrodes 4a, 4b which are close to the heating portion 3a are deformed and there is a possibility that it gives rise to the cracks or the peeling-off in the protective layer 5.
When the cracks or the peeling-off are generated in the protective layer 5, it brings about the change of the resistance value of the heating resistor 3 and hence, there arises a problem that the printing quality and the printing lifetime of the thermal head are lowered.
The present invention has been made in view of the above mentioned problems, and it is an object of the present invention to provide a thermal head having a long lifetime and a high printing quality by decreasing the size of steps of a protective layer formed on a heating portion. It is also an object of the present invention to provide a manufacturing method thereof.
According to a first aspect of the present invention, a thermal head of the present invention which is provided for solving the abovementioned drawbacks includes a heat insulation layer which is formed on a substrate, a plurality of heating resistors which are formed on an upper surface of the heat insulation layer, a plurality of electrodes which are connected to the heating resistors and form heating portions at portions of the heating resistors, and a protective layer which covers surfaces of the heating resistors and the electrodes, wherein the electrodes are made of lower-layer electrodes and upper-layer power feeding layers, wherein the lower-layer electrodes and the upper-layer power feeding layers are dissolved by one etchant, wherein the lower-layer electrodes are formed at positions excluding the heating portions and positions in the vicinity of the heating portions, and wherein the upper-layer electrodes are continuously formed from portions at positions in the vicinity of the heat generating bodies to upper surfaces of the lower-layer electrodes excluding the heating portions.
According to a second aspect of the present invention, a thermal head of the present invention which is provided for solving the abovementioned drawbacks is constituted such that a material which constitutes at least the lower-layer electrodes or the upper-layer electrodes is made of any material selected from a group consisting of aluminum, copper, gold and an alloy of these metals.
According to a third aspect of the present invention, a thermal head of the present invention which is provided for solving the abovementioned drawbacks is constituted such that a film thickness of the upper-layer electrodes is set to a value which falls within a range of 0.1 to 0.3 xcexcm.
According to a fourth aspect of the present invention, a thermal head manufacturing method of the present invention which is provided for solving the abovementioned drawbacks includes a first step in which a heat insulation layer is formed on a substrate, a second step in which a plurality of heating resistors are formed on an upper surface of the heat insulation layer, a third step in which electrodes which are connected to heat resistors are formed, and a fourth step in which a protective layer which covers at least surfaces of the heating resistors and the electrodes is formed, wherein the third step is comprised of a step in which metal films are formed on the heating resistors by patterning so as to form lower-layer electrodes on portions excluding heating portions and portions at positions in the vicinity of the heating portions of the heating resistor and a step in which metal films are continuously formed by patterning from portions at positions in the vicinity of the heating portions to upper surfaces of the lower-layer electrodes excluding the heating portions so as to form upper-layer electrodes from portions at positions in the vicinity of the heating portions to upper surfaces of the lower-layer electrodes excluding the heating portion.
According to a fifth aspect of the present invention, in a thermal head manufacturing method of the present invention which is provided for solving the abovementioned drawbacks, the lower-layer electrodes and the upper-layer electrodes are made of one material.
According to a sixth aspect of the present invention, in a thermal head manufacturing method of the present invention which is provided for solving the abovementioned drawbacks, a material which constitutes at least the lower-layer electrodes or the upper-layer electrodes is any material selected from a group consisting of aluminum, copper, gold and an alloy of these metals.
According to a seventh aspect of the present invention, in a thermal head manufacturing method of the present invention which is provided for solving the abovementioned drawbacks, a film thickness of the upper-layer electrodes is set to a value which falls within a range of 0.1 to 0.3 xcexcm.
According to an eighth aspect of the present invention, in a thermal head manufacturing method of the present invention which is provided for solving the abovementioned drawbacks, the metal films which constitute the upper-layer electrodes are formed into films by a sputtering technique.